This invention relates to Coriolis flowmeters. More particularly, this invention relates to reducing a flag dimension of a Coriolis flowmeter by using flow tubes having a substantially semicircular arc and one set of brace bars. Still more particularly, this invention relates to a configuration of components that maintains zero stability and reduces the amplitude of the vibrating flow tubes to reduce stress applied to the brace bars.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and U.S. Pat. No. Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters have one or more flow tubes of a curved configuration. Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the flow tube or flow tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver applies a force to the flow tube in order to cause the flow tubes to oscillate in a desired mode of vibration. Typically, the desired mode of vibration is a first out of phase bending mode. When no material is flowing through the flowmeter, all points along a flow tube oscillate with an identical phase. As the material begins to flow, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed on the flow tube to produce sinusoidal signals representative of the motion of the flow tube. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes. Electronic components connected to the sensor then use the phase difference and frequencies of the signals to a determine mass flow rate and other properties of the material.
An advantage that Coriolis flowmeters have over other mass flow measurement devices is that flowmeters typically have less than 0.1% error in the calculated mass flow rates of a material. Other conventional types of mass flow measurement devices such as orifice, turbine, and vortex flowmeters, typically have 0.5% or greater errors in flow rate measurements. Although Coriolis mass flowmeters have greater accuracy than the other types of mass flow rate devices, the Coriolis flowmeters are also more expensive to produce. Users of flowmeters often choose the less expensive types of flowmeters preferring to save cost over accuracy. Therefore, makers of Coriolis flowmeters desire a Coriolis flowmeter that is less expensive to manufacture and determines mass flow rate with an accuracy that is within 0.5% of the actual mass flow rate in order to produce a product that is competitive with other mass flow rate measurement devices.
One reason that Coriolis meters are more expensive than other devices is the need for components that reduce the number of unwanted vibrations applied to the flow tubes. One such component is a manifold which affixes the flow tubes to a pipeline. In a dual tube Coriolis flowmeter, the manifold also splits the flow of material received from a pipeline into two separate flows and directs the flows into separate flow tubes. In order to reduce the vibrations caused by outside sources, such as a pump, that are connected to the pipeline, a manifold must have a stiffness that is sufficient enough to absorb the vibrations. Most conventional manifolds are made of cast metal in order to have a sufficient mass. Furthermore, there is a spacer between the manifolds that maintains the spacing between inlet and outlet manifolds. This spacer is also made out of a metal or other stiff material in order prevent outside forces from vibrating the flow tubes. The large amount of metal used to create these castings increases the cost of the flowmeter. However, the elimination of unwanted vibrations greatly increases the accuracy of the flowmeters.
A second problem for those skilled in the Coriolis flowmeter art is that flowmeters may have a flag dimension that is too big to be used in certain applications. For purposes of this discussion, flag dimension is the length that a flow tube loop extends outward from a pipeline. There are environments where space is constrained or is at a premium. A flowmeter having a typical flag dimension will not fit in these confined areas.
It is a particular problem to reduce the flag dimension of flow tubes in a Coriolis flowmeters that handle large flow rates. For purposes of this discussion, large flow rates are 700 lbs./minute or greater. One reason that reducing the flag dimension is a problem in flowmeter handling larger flow rates is that the flow tubes must have larger diameters. Larger diameter flow tubes have higher drive frequencies than smaller diameter flow tubes and are harder to design when reducing the flag dimension. The larger diameter of the flow tube also causes zero stability problems when a smaller flag dimension is created. For these reasons, it is a particular problem to create a dual flow tube Coriolis flowmeter capable of handling large flow rates.
The above and other problems are solved and an advance in the art is made by the provision of a Coriolis flowmeter having a reduced flag dimension in the present invention. The Coriolis flowmeter of the present invention has flow tubes that are capable of handling large mass flow rates. The Coriolis flowmeter of the present invention does not have a conventional manifold and spacer. Instead, the spacer substantially surrounds the manifolds. This configuration reduces the cost of the flowmeter. The Coriolis flowmeter of the present invention also has a reduced flag dimension which allows the Coriolis flowmeter of the present invention to be used in areas where space is at a premium and it would be impossible to use a conventional Coriolis flowmeter having a conventional flag dimension.
The flag dimension of the flow tube is reduced by forming the flow tubes in a semicircle between inlet ends and outlet ends of the flow tubes. The semicircle shape of the flow tubes reduces the rise of flow tube to reduce the flag height. In order to increase the accuracy of the flowmeter, the entire length of the semicircle must vibrate.
A driver is affixed to the flow tubes at a position along each flow tube that is substantially perpendicular to a plane containing the inlet end and the outlet end of the flow tube. The driver is positioned at this point to minimize the amount of energy applied to the flow tubes by the driver to cause the flow tubes to oscillate. Drive signals are applied to the driver to cause the driver to oscillate the flow tubes at a low amplitude to reduce the stress applied to brace bars affixed to the flow tubes. The driver must also drive the flow tubes to vibrate at a frequency that is higher than conventional flow tubes.
To separate vibrations modes in the flow tube while the flow tube is being oscillated, a first brace bar is affixed to the flow tubes proximate the inlet ends and a second brace bar is affixed to the flow tubes proximate the outlet ends. Brace bars are metal components that are affixed to each of the flow tubes at substantially the same location along the flow tubes.
In order to sense Coriolis effect in the oscillating flow tubes, the pick-off sensors have to be affixed to the flow tubes in a position that allows the sensors to detect the greatest amount of Coriolis force at a low amplitude vibration. This allows a lower amplitude vibration to be used in order to reduce the stress applied to the brace bar.
An inlet manifold and an outlet manifold may be affixed to the inlet and outlet ends of the flow tube to connect the flow tubes to a pipeline. Each manifold is a separate component that is cast separately to reduce the cost of material. Each manifold may have a flow path that bends substantially 90 degrees to connect the inlet and outlet ends of the semicircular arc to a pipeline.
A spacer is affixed to each of the manifolds to maintain the distance between the manifolds. The spacer is a structure having four sides with opposing ends affixed to the inlet and outlet manifolds. The spacer encloses a hollow cavity. This reduces the amount of material used in casting the manifold. Openings in the top side of the spacer allow the manifold to connect to the semicircular arc of the flow tubes which protrude outward from the spacer.
A casing may be affixed to the top side of the spacer to enclose the flow tubes. It is a problem that the casing may resonate a frequency that is close to the frequency of the vibrating flow tubes. This may cause inaccuracies in the readings of properties of material flowing through the flow tube. To change the resonant frequency of the case, a mass may be affixed to the casing to change the resonant frequency of the case.